1. Field of the Invention
The present invention relates to a pneumatic tire capable of reinforcing a sidewall portion and enhancing durability, steering stability and noise reduction ability.
2. Description of the Related Art
As shown in FIG. 5, a pneumatic tire usually includes a pair of annular bead portions 1, sidewall portions 2 extending from the respective bead portions 1 radially outward of the tire, and tread portions 4 connected to the respective sidewall portions 2 radially outward of the tire through a shoulder portion 3. A carcass ply 5 forming a tire frame is disposed such as to extend between the bead portions 1. The sidewall portion 2 is largely bent during running of a vehicle, but its thickness is relatively small, and there is a tendency that the durability, the steering stability and the noise reduction ability are deteriorated. To enhance them, it is preferable that the sidewall portion 2 is reinforced, but even if the rubber hardness is enhanced, the obtained effect is limited.
Hence, Japanese Patent Application Laid-open No. H8-197917 discloses a pneumatic tire in which the sidewall portion is provided with a plurality of ribs extending in an inclined direction with respect to a circumferential direction of the tire. According to the pneumatic tire, the sidewall portion can be reinforced over the substantially entire region of its cross section height. However, since a design of keynote of the outer surface of the sidewall portion is different from that of the tread surface, discontinuous portion of the reinforced region is generated in the vicinity of the shoulder portion in some cases, and the durability may be deteriorated in this discontinuous portion in some cases. If the sidewall portion is reinforced, the vibration can be suppressed and noise can be reduced, but frequency band of suppressed vibration is relatively narrow, and reduction effect of road noise is poor.
Japanese Patent Application Laid-open No. H11-291718 discloses a pneumatic tire in which a concave block extending from the shoulder portion to the sidewall portion is disposed in the circumferential direction of the tire. According to such a pneumatic tire, since a cross section height of the concave block from the tire equator point is set ½ or less of the tire cross section height, it is not possible to reinforce a thin portion of the sidewall portion exceeding ½ of the tire cross section height, and the effect for enhancing the durability is poor. Further, since the cross section height of the concave block is uniform, there is a problem that an interface of the reinforced region is formed along the circumferential direction of the tire and a crack is prone to be generated along the circumferential direction of the tire due to the sidewall portion is bent. Similarly, since the frequency band of vibration to be suppressed by the reinforcing is relatively narrow, the effect for reducing the road noise is poor.
Japanese Patent Application Laid-open No. 2000-16031 discloses a pneumatic tire in which an inclined groove extending in a direction which is inclined with respect to the circumferential direction of the tire is extended to a lower region of the sidewall portion. However, this structure is for overcoming outer appearance inconvenience of the tire, and is not for reinforcing the sidewall portion.